If you look at a picture of Earth taken from space, the only lines you'll see are mountain ranges and rivers and perhaps a few highways. But if you look at a map, you'll see lines galore: the borders between countries and states, lines of latitude and longitude, and a lot more.
The same thing applies to the sky. It has its own borders, for example -- the boundaries between constellations. It has lines of latitude and longitude. And it even has an equator -- the projection of Earth's equator on the sky.
If you're standing on Earth's equator, the celestial equator stretches from east to west directly across the top of the sky, neatly dividing the sky into northern and southern hemispheres. Best of all, with the exception of a tiny region around the poles, you can see all of both hemispheres. In other words, you can see the entire span of the cosmos.
From the rest of the globe, though, the view is a little different. From here in the United States, for example, the equator arcs across the southern half of the sky. That means that we can see the entire northern celestial hemisphere, but only part of the southern one.
The celestial equator helps define the seasons. The moment the Sun crosses the equator from south to north marks the beginning of spring in the northern hemisphere. And the moment it crosses in the other direction marks the beginning of autumn -- a changing of seasons marked by an invisible line across the sky.
Script by Damond Benningfield, Copyright 2009
For more skywatching tips, astronomy news, and much more, read StarDate magazine.